The opposite of "salty" in this context would be "sweet" (for the buyer), that is cheap.
"Salty" (especially in excess), implies "unpleasant," which (for a buyer) in turn implies "expensive."
You need to differentiate the noun from the verb, then investigate the verb for your answer.
The noun doesn't help much here.
root n.: the part of a plant, usually below the ground, that lacks nodes, shoots, and leaves, holds the plant in position, draws water and nourishment from the soil, and stores food
Origin: Middle English rote from Late Old English from Old Norse rot, akin to Old English wyrt, German wurzel from Indo-European base an unverified form wrād-, twig, root from source Glassical Greek rhiza, Classical Latin radix, root, ramus, branch
root vi.: to give audible encouragement or applause to a contestant or team; cheer. See Synonyms at applaud; to lend support to someone or something.
Origin: possibly alteration of rout. A second source also mentioned rout as the possible origin. root:Possibly an alteration of rout (“to make a loud noise”), influenced by hoot
rout to bellow, used of cattle. (First Known Use: 14th century): 14th century (Middle English rowten, from Old Norse rauta; akin to Old English rēotan to weep, Latin rudere to roar)
Best Answer
Apparently it refers to the salty water of the seas, according to Etymonline. Old refers to veteran, experienced:
Old salt:
Green’s Dictionary of Slang gives usage examples from 1830: